Rapid improvement of portable electronic devices, such as cellphones and laptops, leads to requirements for higher capacity of batteries used for main powder supplies and uninterruptible power supplies of such portable devices. Then, electrochemical devices which are nonaqueous electrolytic batteries, such as lithium ion secondary batteries, have attracted attention because these batteries have a higher energy density than nickel-cadmium batteries and nickel-hydrogen batteries.
Typical examples of the electrolytic solution for lithium ion secondary batteries include nonaqueous electrolytic solutions prepared by dissolving an electrolyte (e.g., LiPF6, LiBF4, LiN(CF3SO2)2, LiCF3(CF2)3SO3) in a solvent mixture of a high permittivity solvent (e.g., ethylene carbonate, propylene carbonate) and a low viscosity solvent (e.g., dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate).
Negative electrode active materials of lithium ion secondary batteries mainly comprise a carbonaceous material which can occlude and release lithium ions. Typical examples thereof include natural graphite, artificial graphite, and amorphous carbon. Further, metal- or alloy-based negative electrodes comprising elements such as silicon and tin are also known to provide a much higher capacity. Positive electrode active materials of the above batteries mainly comprise a transition metal complex oxide which can occlude and release lithium ions. Typical examples of the transition metal include cobalt, nickel, manganese, and iron.
Such lithium ion secondary batteries comprise highly active positive and negative electrodes. These electrodes disadvantageously cause side reactions with the electrolytic solution, and such side reactions are known to decrease the charge and discharge capacities. In order to improve the above battery characteristics, researchers have performed various studies on nonaqueous solvents and electrolytes.
Patent Literature 1 proposes to use an electrolytic solution comprising an organic compound having two or more nitrile groups. The nitrile groups are polarized to give a large dipole moment, and this large dipole moment restrains oxidative decomposition of the electrolytic solution on the positive electrode during charging at high voltage, thereby improving the battery characteristics.
Patent Literature 2 discloses an agent for forming a film on electrode surfaces, the agent comprising a specific nitrile compound, and thus improving the thermal stability of batteries.
Patent Literature 3 discloses a nonaqueous electrolyte secondary battery which comprises a fluorinated nitrile compound in an electrolytic solution, and thus has excellent charge and discharge efficiency and storage characteristics.
Patent Literature 4 discloses that addition of a compound having an isocyanate group to a nonaqueous electrolytic solution restrains a decomposing reaction of a solvent on the negative electrode, and thus improves the cycle characteristics of batteries.
Patent Literature 5 proposes to form a complex of an aliphatic nitrile compound with the surface of a positive electrode active material and thereby form a protective film on the positive electrode. This improves the safety of batteries against overcharge and/or physical impact from the outside.
Patent Literature 6 proposes addition of a sulfate as an additive to a nonaqueous electrolytic solution with an aim of improving the pulse discharge characteristics of an alkali metal electrochemical cell, in particular a primary lithium electrochemical cell.
Patent Literature 7 proposes use of a sulfonate-based compound containing at least one substituent selected from the group consisting of a cyano group, an isocyanate group, a thiocyanate group, and an isothiocyanate group with an aim of improving the high-temperature lifespan characteristics of lithium batteries.
Patent Literature 8 proposes use of a sulfate compound having a C(sp)-C(sp3) unsaturated hydrocarbon bond with an aim of improving the high temperature cycle characteristics of lithium batteries.